1. Field of the Invention
This invention relates to the testing in microlithography for the production of circuits having a very high integration density.
2. Description of the Prior Art
The production of circuits such as these involves several stages which differ according to the methods used. In photolithography, optical or electronic drawing machines produce plates carrying a single pattern, generally on a scale of 10, which will be referred to hereinafter as the reticle. A photorepeater then produces a mask carrying a regular matrix of identical patterns reproducing the pattern of the reticle on a scale of 1 in the form of opaque zones and transparent zones. This mask enables certain parts of a semiconductor substrate coated with a photosensitive resin to be selectively exposed. In a variant of this method which is known as direct photorepetition, the pattern may be directly transferred from the reticle to the substrate coated with photoresist without using an intermediate mask. The present invention is particularly intended for detecting bugs in the reticle and in the mask. It enables a rapid analysis to be made with a resolution of less than 1 .mu.m. The position of the bug does not have to be perfectly defined. Accordingly, it is sufficient for an operator to be able to locate the bug in the field of a microscope in view of repairing it.
Reticles may have the following bugs:
errors in the positioning of the pattern elements relative to the alignment reference marks;
too few or too many pattern elements;
distorsion of the form of the pattern or incorrect dimensions;
errors in design and handling;
treatment faults (holes, excessive or inadequate etching irregularity).
Masks on a scale of 1 obtained from a theorically faultless reticle may in turn present bugs of the same kind. Additional bugs may arise, particularly in the positioning of the image, rotation of the image, enlargement, distorsion, etc.
Tests have shown that the error level and lack of rapidity of testing by visual observation, for example using microscopes, become prohibitive for circuits having a high integration density. There are techniques for scanning masks by a quasi-spot beam of electrons or photons, but unfortunately they lead to great number of data which are difficult to process.